Digital cameras comprise imaging optics which are adapted to focus the image of a target onto an image sensor. The image sensor comprises an array of light sensitive pixels. The plurality of signals provided by the individual pixels constitute the digital image of the target. To the first approximation, a signal provided by each individual pixel is proportional to a time period between resetting or activation of said pixel and shifting of said signal (e.g. charge) to a shift register or to a memory.
A typical low-cost image sensor does not comprise charge shift registers and it cannot hold and store the signals. Thus, when the reading of all signals can not take place simultaneously, the signals obtained from the individual pixels correspond to different time intervals. This may lead to a distortion of the recorded image, image artifacts, especially when the camera or the target is moving.
The problem may be remedied by controlling the optical exposure of all pixels substantially simultaneously by using a so-called global shutter. The signal provided by an individual pixel remains substantially constant after the closing of the global shutter, and the obtained image corresponds to simultaneous exposure even though the electrical reading of the pixels would not take place simultaneously.
Traditional mechanical global shutters exhibit several drawbacks, e.g. complex mechanical structure, large size and relatively high power consumption. Therefore, mechanically implemented global shutters are not optimal for imaging systems integrated in small portable devices.
U.S. Pat. No. 4,249,814 discloses a camera shutter which employs the principles of total internal reflection as a means for controlling transmission of light to a camera film. The shutter comprises at least two prism elements. Total internal reflection is enabled or disabled by mechanically opening or closing a gap between said prisms.
U.S. Pat. No. 6,377,383 discloses an optical switch based on total internal reflection. A stiffened surface portion, e.g. a membrane, is suspended in a liquid. The stiffened surface portion is moved electrostatically to enable or disable total internal reflection at an interface.
U.S. Pat. No. 4,701,021 discloses a light beam intensity modulator for controlled attenuation of a light beam. The modulator comprises a capillary space between two prisms. An amount of fluid is displaced electrostatically in said capillary space to enable or disable total internal reflection at an interface between the fluid and the wall of the capillary space. When the total internal reflection is enabled by displacing the fluid away from the capillary space, the reflected light is directed towards an absorbing screen, and the transmitted portion of light is substantially attenuated.